1. Field of the Invention
The present invention relates to cargo box extensions generally and more particularly to cargo box extensions for selectively increasing the height of existing cargo box walls and thereby expand the useful area of the cargo box. Even more particularly the current invention relates to cargo box extensions having a plurality of lift arm mechanisms connected together by a closed loop drive system.
2. Description of the Prior Art
Various devices have been used in the past to selectively increase the cargo area of a cargo box, however these devices include many common disadvantages. Prior to the current invention, various lifting mechanisms were utilized to raise tops or walls, but the lifting mechanisms were powered individually or at best in pairs. This allowed some of the lifting mechanisms to advance faster than others such that there was an uneven lifting or retraction of the extension which often allowed the device to bind and to jam. For example U.S. Pat. No. 4,703,971 issued to Schmeichel et al. shows a cargo box extension having multiple lift mechanisms each of which requiring independant manual operation.
In addition, the prior art devices had many of the working parts contained inside the cargo area where they were difficult to reach, repair or protect from the cargo in the box.